Thanks for the RFC
If you develop a feature that need to be hidden, don't do it
The documentation should be clear about the type of use case for this. And that it's an advanced extension points.
Go to Settings.yaml, try to avoid Package.php has much as possible ... impossible to overload from an other package, ... And convention is configuration go to Settings.yaml.
An other option, can be an Interface + Annotations + StaticCompile.
As said in Slack, I love the ES6 tagged template, but nothing blocking from my side to use something else (love the nerd assembly based proposal).
also, dont expose too much internals.
When this new feature is rock solid, fine to move EEL handling on top of it, but first thing first